Stricter performance criteria for on-board fuel vapor recovery systems of automotive vehicles impose greater criticality on system components, such as canister purge solenoid (CPS) valves. A typical control strategy for a CPS valve comprises energizing its solenoid by means of a pulse width modulated signal from an electrical control circuit. The extent to which the CPS valve opens is related to the duty cycle of the modulation, and this can be portrayed graphically by plotting flow through the CPS valve as a function of duty cycle. Ideally, this relationship would be exactly linear over the full operating range, but in fact such is not the case for certain existing CPS valves in which the armature motion is largely underdamped and the armature and stator have flat confronting faces at the air gap that separates them.
Within the lower duty cycle range, an irregular change may occur in the flow characteristic. For mass-produced CPS valves, the exact duty cycle at which this irregular change occurs is apt to vary from valve to valve, and consequently the possibility of such variance in any given valve may detrimentally affect compliance of the vapor recovery system in which it is installed with relevant specifications.
The present invention arises in part through the recognition that this irregular change is a naturally occurring phenomenon in a CPS valve whose armature and stator have flat confronting faces at the air gap that separates them, and whose armature motion is largely underdamped. As the armature approaches the stator, it causes an increase in the inductance of the solenoid because more of the armature is placed in the magnetic circuit. The increase in inductance causes the solenoid's impedance to increase, slowing the solenoid's response to a pulse width modulated signal and attenuating the magnetic attraction force exerted on the armature. Underdamped armature motion results in more sudden change in solenoid inductance, and it is such type of change that is believed to give rise to the irregular change.
The present invention provides a solution that substantially eliminates such irregularity. The solution comprises two components: 1) the armature-stator interface is designed with a "step" to impart more damping to the armature motion; and 2) the armature is dimensioned in relation to a tube that guides its motion such that the armature motion is also pneumatically damped. In summary, limiting the rate at which the armature can move limits the rate of change of the solenoid's inductance, and this significantly attenuates the aforementioned irregularity such that it may be considered substantially eliminated.
A further benefit of imparting increased damping to the armature motion is a reduction in the ripple content of the purge flow through the CPS valve. The flow through a typical pulse width modulated CPS valve contains a ripple component due to the fact that the armature is being modulated. Peaks of these ripples can have an adverse effect on a system's compliance with relevant specifications. Increased damping reduces the peak amplitude of the ripple excursions. The resulting improvement is especially significant during the generally lower flow rates occurring at engine idle.
A still further improvement is obtained by incorporating a regulator into the CPS valve and providing the regulator with a tapered pintle type regulator valve element.
The known state of the art is represented by the following U.S. Pat. Nos. 4,700,750; 4,867,126; 4,901,702; 4,901,974; and 4,951,637.
The foregoing features, advantages, and benefits of the invention, along with those already mentioned, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a presently preferred embodiment of the invention according to the best mode contemplated at this time for carrying out the invention.